Intermetallic nanoparticles

ABSTRACT

A process for preparing intermetallic nanoparticles of two or more metals is provided. In particular, the process includes the steps: a) dispersing nanoparticles of a first metal in a solvent to prepare a first metal solution, b) forming a reaction mixture with the first metal solution and a reducing agent, c) heating the reaction mixture to a reaction temperature; and d) adding a second metal solution containing a salt of a second metal to the reaction mixture. During this process, intermetallic nanoparticles, which contain a compound with the first and second metals are formed. The intermetallic nanoparticles with uniform size and a narrow size distribution is also provided. An electrochemical device such as a battery with the intermetallic nanoparticles is also provided.

GOVERNMENT INTERESTS

This invention was made with Government support under Contract No.DE-AC02-06CH11357 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

FIELD

The technology generally relates to the field of intermetallicnanoparticles of two or more metals.

BACKGROUND

Intermetallic nanoparticles are desired for various applications becausethey may exhibit useful properties such as high thermal conductivity,high heat capacity, high electrical conductivity, etc. Typically,intermetallic nanoparticles are prepared by attrition milling of thebulk intermetallic compound or co-precipitation techniques that mayrequire processing at high temperatures. In addition, the nanoparticlesthat are prepared by milling are not uniform and exhibit a largevariation in size.

Since their first commercialization in the 1990s, rechargeablelithium-ion (Li-ion) batteries have served as major power sources for awide range of electronic products. In recent years, an increase inglobal energy demand, rising and fluctuating crude oil prices, andenvironmental concerns have led to an increase in demand for Li-ionbatteries. In particular, Li-ion battery technology is being developedfor applications in electric vehicles (EVs), hybrid electric vehicles(HEVs), and plug-in hybrid electric vehicles (PHEVs). For suchapplications, improved Li-ion battery anodes from intermetallicnanomaterials providing high energy density and high power capacity aredesired.

SUMMARY

In one aspect, a process is provided including the steps of: dispersingnanoparticles of a first metal in a solvent to prepare a first metalsolution; forming a reaction mixture comprising the first metal solutionand a reducing agent; heating the reaction mixture to a reactiontemperature; and adding a second metal solution comprising a salt of asecond metal to the reaction mixture whereby intermetallic nanoparticlesare formed, where the intermetallic nanoparticles comprises a compoundcomprising the first metal and second metal, and the first and secondmetal are not the same. In some embodiments, the process is conductedunder an inert atmosphere. In some embodiments, the first metal is Sn,Pb, In, Ga, Bi, Ge, Zn, Ni. In some embodiments, the second metal is Cu,Ag, Au, Pt, Pd, Ru, Ir, Fe, Co, Ni. In some embodiments, the salt of thesecond metal may contain sulfates, nitrates, chlorides and acetates suchas copper(II) sulfate, copper(II) nitrate, copper(II) chloride,copper(II) acetate, etc. In some embodiments, the reducing agentcontains NaHPO₂, NaBH₄, LiAlH₄, citric acid, ascorbic acid, tetrakis(dimethylamine) ethylene.

In some embodiments, the process further includes exposing theintermetallic nanoparticles to a third metal solution comprising a saltof a third metal, whereby the ternary intermetallic nanoparticles areformed comprising a compound comprising the first metal, the secondmetal and the third metal and where the third metal is not the same asthe first metal or the second metal.

In some embodiments, the third metal is Ag, Au, Pt, Pd, Ru, Ir. In someembodiments, the reaction temperature is between about 50° C. to about200° C. In some embodiments, the reaction temperature is between about100° C. to about 150° C. In some embodiments, the process furtherincludes cooling the reaction mixture.

In some embodiments, the crystal structure of the intermetallicnanoparticles differs from the crystal structure of the first metal andthe second metal. In some embodiments, the intermetallic nanoparticleshave an average diameter from about 100 nm to about 400 nm. In someembodiments, the intermetallic nanoparticles have a polydispersity ofabout 0.2. In some embodiments, the process further includes adding theintermetallic nanoparticles to a heat transfer fluid to form a heattransfer mixture whereby the thermal conductivity of the heat transfermixture is greater than the thermal conductivity of the heat transferfluid.

In one aspect, intermetallic nanoparticles are provided having Sn andCu, where the intermetallic nanoparticles contain a compound comprisingSn and Cu, and the average diameter of the intermetallic nanoparticlesis from about 100 nm to about 400 nm. In some embodiments, theintermetallic nanoparticles have a polydispersity less than 0.5. In someembodiments, the intermetallic nanoparticles have a polydispersity ofabout 0.2.

In some embodiments, an electrode containing the intermetallicnanoparticles is provided. In some embodiments, a battery comprising theelectrode is provided. In some embodiments, a heat transfer fluidcontaining the intermetallic nanoparticles is provided.

In one aspect, an electrochemical device containing an anode, a cathodehaving intermetallic nanoparticles with Sn and Cu, where theintermetallic nanoparticles is a compound of Sn and Cu, and the averagediameter of the intermetallic nanoparticles from about 100 nm to about400 nm; and an electrolyte. In some embodiments, the electrochemicaldevice is a lithium-ion battery. In some embodiments, the capacity ofthe battery is greater than 80 mA/g at 0.5 V during the discharge cycle.

Additional features, advantages, and embodiments of the presentdisclosure may be set forth from consideration of the following detaileddescription, drawings, and claims. Moreover, it is to be understood thatboth the foregoing summary of the present disclosure and the followingdetailed description are exemplary and intended to provide furtherexplanation without further limiting the scope of the present disclosureclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects, features, and advantages ofthe disclosure will become more apparent and better understood byreferring to the following description taken in conjunction with theaccompanying drawings, in which:

FIGS. 1A and 1B is a schematic of the apparatus used for preparingintermetallic nanoparticles.

FIG. 2A is a scanning electron microscope (SEM) image of Cu₆Sn₅ productof Example 2.

FIG. 2B is a SEM image of Cu₆Sn₅ product of Example 2.

FIG. 2C is a SEM image of Cu₆Sn₅ product of Example 2.

FIG. 2D is a SEM image of Cu₆Sn₅ product of Example 2.

FIG. 2E is a SEM image of Cu₆Sn₅ product of Example 2.

FIG. 3A is a SEM image of Cu₃Sn, product of Example 2.

FIG. 3B is an elemental distribution map of Cu using Energy DispersiveX-ray (EDAX) spectroscopy.

FIG. 3C is an elemental distribution map of Sn using EDAX spectroscopy.

FIG. 4A is an X-ray diffraction (XRD) pattern of Cu₃Sn intermetallicnanoparticles.

FIG. 4B is an XRD pattern of Cu₆Sn₅ intermetallic nanoparticles.

FIG. 5A depicts the particle size distribution (curve A) of Cu₆Sn₅intermetallic nanoparticles using Dynamic Laser Scattering data. Thesecond curve (B) shows volumetric percentage of the particles of a givensize, i.e. indicates that most particles are of the average size.

FIG. 5B is an SEM image of Cu₆Sn₅ particles from a commercial source.

FIG. 6 is a graph showing capacity versus voltage of a Li-ion coin cellwith Cu₆Sn₅ intermetallic nanoparticles as the positive electrode duringseveral charge/discharge cycles.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here. It will be readily understood that the aspects of thepresent disclosure, as generally described herein, and illustrated inthe figures, can be arranged, substituted, combined, and designed in awide variety of different configurations, all of which are explicitlycontemplated and made part of this disclosure.

A process is provided for preparing intermetallic nanoparticles using alow-temperature chemical synthesis route. The process includes thesteps: a) dispersing nanoparticles of a first metal in a solvent toprepare a first metal suspension, b) forming a reaction mixture with thefirst metal suspension and a reducing agent, c) heating the reactionmixture to a reaction temperature; and d) adding a second metalsuspension containing a salt of a second metal to the reaction mixture.Intermetallic nanoparticles, which contain a compound with the first andsecond metals are formed by this process.

In some embodiments, the first metal is Sn and the second metal is Cu.In some embodiments, the first metal (M1) is Sn, Pb, Sb, In, Ga, Bi, Ge,Zn, Ni, the second metal (M2) is Cu, Ag, Au, Pt, Pd, Ru, Ir, Fe, Co, Niand the third metal (M3) is Ag, Ag, Au, Pt, Pd, Ru, Ir. Although notbound by theory, it is understood that the standard electrode potential(SEP) of the second and third metal has to be more positive than SEP ofthe first metal for spontaneous redox exchange reaction. In someembodiments, the intermetallic nanoparticles may contain Cu₃Sn, Cu₆Sn₅or M1, M2, M3 wherein x & y is an integer between 1 to 8 and z is aninteger from 0 to 8. One copper atom can be substituted with Ni, Zn, Fe,etc., to form intermetallic nanoparticles such as NiCu₅Sn₅, or FeCu₅Sn₅.These intermetallic nanoparticles have volumetric capacity (mAh/cc) ofalmost 3 times that of graphite.

It is understood that the steps of the inventive process may beconducted in different orders. In some embodiments, the reaction mixturemay be heated before the addition of the reducing agent. In someembodiments, the reducing agent may be added after the addition of thesecond metal solution. In some embodiments, aliquots of the reducingagent and/or the second metal solution may be added. It is alsounderstood that two or more steps of the process may be combined intoone step.

The term “intermetallic” refers to a compound with one solid phasecontaining two or more metallic elements—a first metal (M1), a secondmetal (M2), a third metal (M3), so on, wherein the crystal structure ofthe intermetallic compound differs from the crystal structures of theconstituents. The term “metal” refers to any element that is a goodconductor of electricity and heat and forms cations. In someembodiments, the first metal is Sn, and the second metal is Cu.

As used herein, the term “nanoparticles” refers to particles that have asingle crystallite grain between 10 nm to 500 nm. Individual grains canagglomerate into clusters/agglomerates up to 10 μm in diameter. In someembodiments, the nanoparticles have a diameter between 150 nm to 750 mm.In some embodiments, the nanoparticles have a diameter between 200 nm to500 nm. In some embodiments, the nanoparticles have a diameter betweenabout 100 nm to about 400 mm. The particle sizes of the intermetallicnanoparticles can be controlled by controlling the process conditions,such as size of precursor M1 particles, concentration of M2 and M3 saltsand reducing agent, temperature, pH, and addition of capping agents.

In one embodiment, intermetallic nanoparticles with Sn and Cu areformed. The Sn nanoparticles are dispersed in a solvent to form areaction mixture. In some embodiments, sonication, stirring ormechanical agitation may be used to disperse the Sn nanoparticles. Insome embodiments, a dispersing agent may be used. Although not bound bytheory, the addition of reducing agent prior to addition of the secondmetal solution helps to reduce any existing oxide on the surface of thefirst metal particles. In some embodiments, the reaction mixture may beformed under an inert atmosphere as shown in FIG. 1A. A reducing agentis added to the reaction mixture. The reaction mixture is heated to areaction temperature of 110° C. A solution of copper sulfate was slowlyadded to the reaction mixture and the intermetallic nanoparticles wereformed. In some embodiments, the copper sulfate solution is addeddropwise under an inert atmosphere as shown in FIG. 1B.

In some embodiments, the solvent is an organic solvent or an aqueoussolution. In some embodiments, the solvent may be an organic solventsuch as ethylene glycol, diethylene glycol, tetraethyleneglycol,glycerin, therminols, or an aqueous solution like deionized water,ethanol.

The reducing agent is a chemical that donates electrons in a redoxreaction. In some embodiments, the reducing agent may be phosphinatessuch as NaH₂PO₂, or hydrides such as LiAlH₄, sodium borohydride, citricacid, ascorbic acid, tetrakis (dimethylamine) ethylene.

It is understood that the intermetallic nanoparticles may be furtherprocessed to form ternary intermetallic nanoparticles containing a thirdmetal in a single or multiple steps. A ternary intermetallicnanoparticles is a compound which contains three different metals. Insome embodiments, the mixture of the M2 and M3 metal salts can be addedto the suspension of M1 nanoparticles. In some embodiments, theintermetallic nanoparticles with the first and second metals may beexposed to a third metal solution containing the salt of a third metal.In some embodiments, exposure to the third metal solution may be done byadding it in the reaction mixture after the intermetallic nanoparticlesare formed.

One of the advantages of the present process is that the intermetallicnanoparticles can be formed at a relatively low temperatures rangingfrom about 50° C. to about 200° C. In some embodiment, the reactiontemperature may be between about 100° C. to about 150° C. In someembodiments, the reaction temperature may vary during the formation ofthe intermetallic nanoparticles. In some embodiments, the reactiontemperature is held constant at 110° C. or 140° C.

Although not bound by theory, it is understood that the reactiontemperature plays a role in the formation of the intermetallicnanoparticles. In some embodiments, the reaction temperature was chosenbased on factors such as the melting point of the first and secondmetals, type of the solvent fluid, concentration of metal salt andreducing agent, type of reducing agent, and the difference in standardpotential of M2 and M1. Each of these parameters controls the overallrate of reaction.

Although not bound by theory, it is understood that two chemicalreactions are taking place during intermetallic nanoparticle synthesis:(1) reduction of copper by reducing agent (represented by Equation 1);and (2) displacement of metallic Sn by Cu (represented by Equation 2):Cu²⁺+H₂PO₂ ⁻+H₂O=>Cu⁰+H₂PO₃ ⁻+2H⁺  (1)Sn⁰+Cu²⁺=>Sn²⁺+Cu⁰  (2)

In some embodiments, excess reducing agent may be added to the reactionmixture. Although not bound by theory, it is understood that excessreducing agent removed any surface oxides from Sn nanoparticles andensured the completeness of Cu²⁺ reduction.

After formation, the intermetallic nanoparticles may be separated fromthe reaction mixture by any known method of separation includingcentrifugation or filtration. In some embodiments, the intermetallicnanoparticles may be washed and dried. In some embodiments, the processof separation and/or drying leads to the agglomeration of theintermetallic nanoparticles into larger aggregates. For example, suchaggregates may form during drying before SEM imaging (as shown in FIGS.2A to 2E).

The intermetallic nanoparticles formed have consistent shapes andexhibit a narrow range of sizes. As shown in FIGS. 4A and 4B the XRDpattern showed no major peaks beside the intermetallic phase, which isindicative of homogenous distribution of Sn and Cu atoms within thenanoparticles. In addition, EDAX mapping of Cu₃Sn cluster (in FIGS. 3Band C) also confirmed the homogenous distribution of Sn and Cu. Theparticle size distribution may be measured using Dynamic LightScattering (DLS) technique. DLS can indirectly measure the particle sizedistribution of small particles in a suspension. When light hits smallparticles the light scatters in all directions and the light scatteringalso varies with time due to Brownian motion of the particles. Twoimportant measures of DLS is the average diameter and polydispersity.The value of the average diameter may be obtained from correlationfunction by appropriate mathematical model of particles moving inbrownian motion (as shown in FIG. 5A). The polydispersity value reflectsthe fraction of particles that deviate from average size. As thepolydispersity value reaches zero, the variation in the average diametersize shrinks. In contrast a the polydispersity value approaches 0.5,nanoparticles have a large variation in sizes. Although not bound bytheory, the present process obtains nanoparticles with limited variationin sizes by dispersing the nanoparticles of the first metal and closelycontrolling the addition of the second metal solution concentration ofthe second metal, the concentration of the reducing agent, and thereaction temperature.

The intermetallic nanoparticles of the present invention may be used ina variety of applications including as catalysts (with or withoutsupport), electrode materials for electrochemical devices, as additivesto heat transfer fluids, electrical interconnect material, etc. In someembodiments, the intermetallic nanoparticles may be used in thepreparation of electrodes for use in a wide variety of applicationsincluding, but not limited to, electrochemical cells, batteries, andsuper-capacitors. In some embodiments, the intermetallic nanoparticlesmay be added to a heat transfer fluid to form a heat transfer mixture toenhance heat transfer for use in a variety of applications such as inpower plants, concentrated solar power facilities, etc. The addition ofthe intermetallic nanoparticles in a heat transfer fluid may enhance theheat transfer properties of the mixture by increasing heat capacity andheat conductivity. In some embodiments, the intermetallic nanoparticlesmay be used as electrically conductive materials for interconnects inelectrical wiring.

Some embodiments provide an electrochemical device comprising: acathode; an anode; and a non-aqueous electrolyte. In some embodiments,the electrochemical device may be a battery, a fuel cell, a capacitor,etc. In some embodiments, the electrochemical device is a lithiumsecondary battery (the Li-ion battery). In the Li-ion battery, thecathode may be lithium-based; the anode is a carbon containing theintermetallic nanoparticles; and the anode and cathode are separatedfrom each other by a porous separator. It is understood that the presentintermetallic nanoparticles may be used with various types of supportmaterial such as carbon to form supported catalysts.

All publications, patent applications, issued patents, and otherdocuments referred to in this specification are herein incorporated byreference as if each individual publication, patent application, issuedpatent, or other document was specifically and individually indicated tobe incorporated by reference in its entirety. Definitions that arecontained in text incorporated by reference are excluded to the extentthat they contradict definitions in this disclosure.

The present technology, thus generally described, will be understoodmore readily by reference to the following examples, which are providedby way of illustration and are not intended to be limiting.

EXAMPLES Example 1 Preparation of Cu₃Sn

Metallic tin (Sn) nanoparticles (<100 nm) manufactured by AmericanElements were suspended in ethylene glycol (EG) and sonicated for atleast 10 minutes with Branson 450 Sonifier at 40% load and 50% dutycycle. The Sn nanoparticles were placed in a three-neck round bottomflask (500 ml) as shown in FIG. 1A with continuous mixing by a magneticstirrer under nitrogen gas purge to provide an inert atmosphere andadditional mixing. A 0.075M solution of NaH₂PO₂*H₂O (7.9 g in 150 mL)was added to the flask at the average rate of ˜2 ml/min. The mixture washeated to the reaction temperature of 140° C. A solution of 0.014MCuSO₄*5H₂O (3.52 g in 50 mL) was slowly added to the reaction mixture atan average rate of ˜1 ml/min under nitrogen gas purge as shown in FIG.1B. The reaction was continued for 30 minutes past the addition ofcopper salt solution. Then the heating was terminated, and mixture wascooled to the room temperature with continued stirring and N₂ purge. Theresulting solid product was separated from the reaction mixture bycentrifuging, followed by decanting and washed once with pure EG, and 3times with ethanol and once with acetone. XRD of the product showed thatit was Cu₃Sn in one solid phase with minor impurities.

SEM images of the product from Example 1 is shown in FIG. 3A. Theseimages show the intermetallic nanoparticles after they have aggregatedduring the drying process.

The product from Example 1 was placed on a Si substrate and an elementalanalysis was done using Energy Dispersive X-ray (EDAX) spectroscopy.FIGS. 3B and 3C show the elemental mapping of Cu and Sn, respectively.FIG. 3A is the SEM image of the same intermetallic nanoparticleaggregate. Cu and Sn are distributed fairly evenly within theintermetallic nanoparticles as shown in FIGS. 3B and 3C.

The product from Example 1 was characterized with X-ray diffraction(XRD). The XRD pattern shown in FIG. 4A confirm that the product fromExample 1 is Cu₃Sn because the experimental peaks matches up (bottomgraph) with the peaks reported in the Power Diffraction (ICDD) database(03-065-4653).

Example 2 Preparation of Cu₆Sn₅ the Same Procedure was Followed as inExample with Modifications Listed in Table 1

TABLE 1 Example 1 and 2 Sn nanoparticles NaH₂PO₂*H₂O CuSO₄*5H₂O in 100ml of in 150 ml of in 50 ml of Reaction Product Example Product EG EG EGtemperature weight 1 Cu₃Sn 1.18 g 7.9 g 3.52 g 140° C. 2.5 g  (0.01M)(0.075M) (0.014M) 2 Cu₆Sn₅  2.9 g 7.9 g 3.99 g 110° C. 2.9 g (0.024M)(0.075M) (0.016M)

The product from Example 2 was characterized using X-ray diffraction(XRD). The XRD pattern shown in FIG. 4B confirms that the product fromExample 1 is Cu₆Sn₅ because the experimental peaks match up with thepeaks reported in the ICDD (00-045-1488) database (top base).

The product from Example 2 was dispersed in deionized water and DynamicLaser Scattering (DLS) technique was used to determine the particle sizedistribution. The results of DLS shown in FIG. 5A indicates that theaverage diameter for the Cu₆Sn₅ nanoparticles was ˜360 nm. In addition,the Cu₆Sn₅ nanoparticles have a relatively narrow range of distributionas evidenced by a polydispersity value of 0.21. This indicates that theintermetallic nanoparticles prepared by this method had a uniform size.In comparison, commercially available Cu₆Sn₅ nanoparticles are larger(several microns) and have a large distribution of sizes as shown inFIG. 5B (SEM image of Cu₆Sn₅ nanoparticles produced by WildcatTechnologies).

Example 3 Preparation and Testing of electrodes with Cu₆Sn₅

Electrodes were prepared with 80 wt. % of the product from Example 2, 10wt. % acetylene black as the conductive agent, and 10 wt. %polyvinylidene difluoride binder. The loading density of the activematerial was around 4.8 mg/cm². The electrolyte was 1.2M LiPF₆ dissolvedin a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC)in a 3:7 volume ratio.

CR2032-type coin cells (1.6 cm²) were assembled with lithium metal asthe negative and the electrode above as the positive electrode. Thecells were discharged first to 0V and then charged to 2.5 V. Thereafterthe cells were discharged and charged between 0 and 2.5 V for severalcycles under the current density of 20 mA/g. The capacity in MAh/g wasmeasured at different voltages during several charge/discharge cycles isshown in FIG. 6. The Li-ion battery tested with the Cu₆Sn₅ nanoparticlesshowed superior performance as compared to values in the literature.During the discharges (decreasing voltage), the capacity at 0.5V wasbetween 100 to 150 mAh/g. It is understood that the smaller and nearlyuniform sizes of the intermetallic nanoparticles and the narrow sizedistribution leads to superior performance.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the elements (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Additionally, the terms andexpressions employed herein have been used as terms of description andnot of limitation, and there is no intention in the use of such termsand expressions of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the invention claimed.Additionally the phrase “consisting essentially of” will be understoodto include those elements specifically recited and those additionalelements that do not materially affect the basic and novelcharacteristics of the claimed invention. The phrase “consisting of”excludes any element not specifically specified.

The present disclosure is not to be limited in terms of the particularembodiments described in this application. Many modifications andvariations can be made without departing from its spirit and scope, aswill be apparent to those skilled in the art. Functionally equivalentmethods and apparatuses within the scope of the disclosure, in additionto those enumerated herein, will be apparent to those skilled in the artfrom the foregoing descriptions. Such modifications and variations areintended to fall within the scope of the appended claims. The presentdisclosure is to be limited only by the terms of the appended claims,along with the full scope of equivalents to which such claims areentitled. It is to be understood that this disclosure is not limited toparticular methods, reagents, compounds compositions or biologicalsystems, which can, of course, vary. It is also to be understood thatthe terminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting.

In addition, where features or aspects of the disclosure are describedin terms of Markush groups, those skilled in the art will recognize thatthe disclosure is also thereby described in terms of any individualmember or subgroup of members of the Markush group.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeinclude the number recited and refer to ranges which can be subsequentlybroken down into subranges as discussed above. Finally, as will beunderstood by one skilled in the art, a range includes each individualmember.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A process comprising: dispersing Sn nanoparticlesin a solvent to prepare a first metal suspension; forming a reactionmixture comprising the first metal solution and a reducing agentselected from the group consisting of phosphinates and hydrides;adjusting the reaction mixture to a reaction temperature between about50° C. and 200° C.; and adding a second metal solution comprising a saltof Cu to the reaction mixture; forming intermetallic nanoparticles areformed by reaction, the intermetallic nanoparticles comprising Sn andCu.
 2. The process of claim 1, wherein the process is conducted under aninert atmosphere.
 3. The process of claim 1, wherein the Cu saltcomprises sulfates, nitrates, chlorides, and acetates.
 4. The process ofclaim 1, wherein the reducing agent is selected from the groupconsisting of NaH₂PO₂, LiAlH₄, sodium borohydride, citric acid, andascorbic acid.
 5. The process of claim 1, wherein the reactiontemperature is between about 50° C. to about 200° C.
 6. The process ofclaim 1, wherein the reaction temperature is between about 100° C. toabout 150° C.
 7. The process of claim 1, further comprising cooling thereaction mixture to about room temperature occurs about 30 minutes afteradding the Cu metal solution.
 8. The process of claim 1, wherein acrystal structure of the intermetallic nanoparticles differs from acrystal structure of the Sn metal and a crystal structure of the Cumetal.
 9. The process of claim 1, wherein the intermetallicnanoparticles have an average diameter from about 100 nm to about 400nm.
 10. The process of claim 1, wherein the intermetallic nanoparticleshave a polydispersity of about 0.2.
 11. The process of claim 1 furthercomprising adding the intermetallic nanoparticles to a heat transferfluid to form a heat transfer mixture whereby the thermal conductivityof the heat transfer mixture is greater than the thermal conductivity ofthe heat transfer fluid.
 12. A process comprising: dispersing Snnanoparticles in a solvent to prepare a Sn suspension; forming areaction mixture, under an inert environment, comprising the Snsuspension and a phosphinate reducing agent; adjusting the reactionmixture to a reaction temperature between about 100° C. and about 150°C.; and adding a Cu metal solution comprising a salt of Cu to thereaction mixture; agitating the reaction mixture; cooling the reactionmixture to about room temperature; wherein intermetallic nanoparticlesof Cu₃Sn are formed.
 13. The process of claim 12, wherein thephosphinate reducing agent is NaH₂PO₂*H₂O.
 14. The process of claim 12,wherein the salt of copper is CuSO₄*5H₂O.
 15. The process of claim 12,wherein the cooling occurs about 30 minutes after adding the Cu metalsolution.
 16. The process of claim 12, wherein the ratio (by weight) ofSn nanoparticles to CuSO₄*5H₂O is about 1.18 to 3.52 and wherein thereaction temperature is about 140° C.
 17. A process comprising:dispersing Sn nanoparticles in a solvent to prepare a Sn suspension;forming a reaction mixture, under an inert environment, comprising theSn suspension and a phosphinate reducing agent; adjusting the reactionmixture to a reaction temperature between about 100° C. and about 150°C.; and adding a Cu metal solution comprising a salt of Cu to thereaction mixture; agitating the reaction mixture; cooling the reactionmixture to about room temperature; wherein intermetallic nanoparticlesof Cu₆Sn₅ are formed.
 18. The process of claim 12, wherein thephosphinate reducing agent is NaH₂PO₂*H₂O.
 19. The process of claim 12,wherein the salt of copper is CuSO₄*5H₂O.
 20. The process of claim 12,wherein the cooling occurs about 30 minutes after adding the Cu metalsolution.
 21. The process of claim 12, wherein the ratio (by weight) ofSn nanoparticles to CuSO₄*5H₂O is about 2.9 to 3.99 and wherein thereaction temperature is about 110° C.